Much research and clinical investigation is currently focused on the use of stem cells and their differentiated progeny. As an example, there are a variety of therapeutic approaches contemplated and/or being developed that involve the use of embryonic (ES) stem cells and/or their progeny. ES cells are pluripotent cells that have the ability to generate any cell type in a mature organism. The clinical potential for stem cell based therapies is immense (Lawrenz et al. Cytotherapy 6, 212-22 (2004)), including treatments for heart disease, diabetes, Parkinson's disease, and leukemia (Hentze et al. Trends Biotechnol 25, 24-32 (2007); Pei et al. J Biol Chem 284, 3365-9 (2009); Jaenisch et al. Cell 132, 567-82 (2008)). However, stem cells such as ES cells pose the risk of tumor formation upon implantation. For example, as few as two ES cells have been shown to form tumors in severe combined immunodeficiency (SCID) mice (Lawrenz et al. Cytotherapy 6, 212-22 (2004)).